1. Field of the Invention
The present invention relates to a toner for visualizing an electrostatic latent image formed on an image bearing member by a method such as electrophotography and electrostatic recording methods. In addition, the present invention also relates to a developer including a toner, a developing method using a toner and a toner container containing a toner.
2. Discussion of the Background
Electrostatic latent images and magnetic latent images, which are formed on an image bearing member of an electrophotographic image forming apparatus or electrostatic recording apparatus are developed with a toner to be visualized.
For example, in electrophotography visual images are typically formed as follows:    (1) an electrostatic latent image is formed on a photoreceptor;    (2) the electrostatic latent image is developed with a developer including a toner to form a toner image on the photoreceptor;    (3) the toner image is transferred onto a receiving material such as papers; and    (4) the toner image on the receiving material is fixed upon application of heat, etc. to form a hard copy.
Recently, a need exists for an electrophotographic image forming apparatus and a developer therefor, which can produce high quality images. In order to produce high quality images, it is essential for the toner included in a developer to have a sharp particle diameter distribution because each of the toner particles can exhibit uniform performance and thereby fine dot images can be well reproduced.
The toners used for developing electrostatic latent images are colored particles typically including a binder resin, and a colorant, a charge controlling agent and additives which are dispersed in the binder resin. The methods for manufacturing the toners are broadly classified into pulverization methods and suspension polymerization methods.
Pulverization methods typically include the following processes:    (1) mixing a colorant, a charge controlling agent, an offset preventing agent and the like materials with a thermoplastic resin upon application of heat thereto to knead the toner constituents;    (2) cooling the kneaded mixture;    (3) pulverizing the kneaded mixture to form a color powder; and    (4) classifying the color powder to form a toner.
The toners prepared by pulverization methods have fair characteristics. However, the pulverization methods have a drawback in that only limited materials can be used as the toner constituents (particularly, as the binder resin). Namely, the kneaded mixture has to be easily pulverized and classified by conventional low-cost pulverizers and classifiers. From this point of view, the kneaded mixture has to be so brittle as to be pulverized. Therefore, the color powder, which is prepared by pulverizing a kneaded mixture, tends to have a broad particle diameter distribution. In order to prepare toner images having good resolution and half tone properties, the color powder has to be classified so as to have a particle diameter of from 5 to 20 μm. Therefore the toner yield is very low in the classification process.
In addition, it is impossible to uniformly disperse a colorant and a charge controlling agent in a thermoplastic resin when the pulverization methods are used. Uneven dispersion of toner constituents adversely affects the fluidity, developing properties, durability and image qualities of the resultant toner.
In attempting to solve such problems, suspension polymerization methods have been proposed and practically used now. The techniques for manufacturing a toner utilizing a polymerization method are known.
However, the particles of toners prepared by suspension polymerization methods have a spherical form and therefore the toners have a drawback of having a poor cleaning property. When toner images have a low image area share (i.e., the percentage of the area of a toner image in a copy sheet is low), the amount of the toner particles remaining on a photoreceptor is small, and therefore a cleaning problem hardly occurs. However, when toner images have a high image area share (for example, copies of photograph images) are produced or when a toner image remains on a photoreceptor without being transferred to a receiving material due to paper jamming problems or the like, a large amount of the toner particles remains on the photoreceptor, resulting in occurrence of background fouling in the resultant or following images. In this case, when a contact charging roller is used, the toner particles remaining on the photoreceptor contaminate the charging roller, resulting in deterioration of the charging ability of the charging roller.
In attempting to solve such a problem, Japanese Patent No. 2,537,503 (i.e., published Japanese Patent Application No. 63-186253) discloses a method in which resin particles prepared by an emulsion polymerization method are associated to prepare toner particles having an irregular form. However, the toner particles prepared by such an emulsion polymerization method include a large amount of a surfactant on or in the toner particles even after the toner particles are washed with water. Therefore, the resultant toner has poor charge stability when environmental conditions change and in addition the distribution of the charge quantity of the toner particles is broad, thereby causing background fouling in copy images. In addition, the remaining surfactant contaminates the photoreceptor and charging roller, developing roller and the like elements used in image forming apparatus, resulting in deterioration of the abilities of the elements.
In contrast, when a contact heating method using a heating member such as heat rollers is used for the fixing process, the toner particles preferably have a releasability from heating members (this releasability is hereinafter referred to as an offset resistance). The offset resistance of a toner can be improved by including a release agent to the toner such that the release agent is present on the surface of the toner. Published Japanese Patent Applications Nos. 2000-292973 and 2000-292978 disclose a method of improving the offset resistance in which resin particles are not only present in the toner particles but also unevenly present on the surface of the toner particles. However, this method has a drawback in that the lower limit of the fixing temperature range increases, namely the toner has poor low temperature fixability, i.e., energy saving is not satisfactory in the fixing process.
The methods in which toner particles having irregular forms are prepared by associating resin particles prepared by an emulsion polymerization method have the following drawbacks.
When particles of a release agent are associated to improve the offset resistance of the toner, the release agent particles are included inside the toner particles, and thereby the offset resistance of the toner cannot be improved. In addition, since resin particles, release agent particles and colorant particles are randomly mixed and fused to prepare toner particles, the formula of the toner particles varies (i.e., contents of the toner constituents in the toner particles vary) and in addition the molecular weight of the binder resin varies in the toner particles. As a result, each toner particle has different surface properties, and therefore the toner cannot stably produce images having good image qualities for a long period of time. Further, in an image forming system which requires the toner to have a low temperature fixability, a poor fixing problem occurs due to the toner particles on the surface of which the resin particles are unevenly present, and therefore the toner cannot be used therefor because of having a narrow fixing temperature range.
In attempting to control the particle diameter distribution of a toner so as to be sharp, a method having the following steps is proposed:    (1) toner constituents including a binder resin are dissolved or dispersed in a solvent to prepare a solution or dispersion thereof;    (2) the solution or dispersion is added to an aqueous medium including resin particles and a crosslinking agent and/or an elongation agent to react the binder resin with the crosslinking agent and/or elongation agent; and    (3) the solvents are removed from the resultant dispersion to prepare toner particles.
In this case, whether the resin particles are present in the aqueous medium largely influences on the particle diameter distribution.
Published Japanese Patent Application No. 8-254853 discloses a capsule toner in which resin particles are fixed on the surface of a toner core material. However, when the toner is fixed, exudation of the wax included in the capsule toner from the surface thereof is prevented by the resin particles present on the surface, resulting in deterioration of fixing properties of the toner. In addition, since the resin particles are not used for controlling the particle diameter of the toner, the capsule toner does not have a sharp particle diameter distribution.
As mentioned above, there is no toner which can produce high quality images and which has good fixing properties.